This study focuses on neurophysiologic and psychologic measures across the personality states of multiple personality disorder (MPD). In repeated randomized trials, subjects with MPD are tested to determine if reliable state differences in cognitive and physiological processing exist. Additionally, special psychological testing assesses the degree of memory overlap between states. Preliminary results suggest that the personality states differ significantly on a variety of psychological and biological measures. The existing literature is reviewed and a new questionnaire battery has been constructed to further assess the incidence, clinical presentation, etiological and treatment factors in this increasingly recognized and novel syndrome of the existence of multiple discrete personalities within the same individual.